It will be a result tree fragment data type. This isn't a problem if what you want is a number or a string, but if you want to retrieve the information from the XML structure using location paths then you need to convert it to a node-set first.

If you are using JUNOS 9.2 or later then you can use the := operator in place of the = to cause $arpChosen to be a node-set rather then a result tree fragment, if earlier you'll have to use the node-set() function instead. You can then do location paths to extract the information that you want. But, keep in mind that the resultant XML structure might be a little different than you expect. The $arpChosen variable will be a node-set with a single node - the root node of your XML structure.

For example, I'm guessing that the correct location path you'd want would be:

$arpChosen/arp-table-entry/ip-address

For more details, you should find the "Applying JUNOS Automation" Day One Guide useful. The PDF can be downloaded for free here: